What is it that divides us, that drives the divisiveness in our political discourse? That's the question from Robert Putnam, Harvard professor and the bestselling author of American Grace.

Is it just our political identity, or does our religious faith — or lack thereof — play a significant role?

"We were interested in the ways that religion either contributes to or does not contribute to, or even detracts from, America's democratic community," Putnam says in a phone interview preceding his appearance at Colorado College.

With his research partner and co-writer, David Campbell, the 69-year-old Putnam — best known as the author of Bowling Alone: The Collapse and Revival of American Community — found that religion plays a smaller role than you might expect.

"America is very unusual because we are both devout and diverse," he says. "And as you can tell by glancing at history, or by glancing around the world today, when religion is taken in high doses, it is often toxic: think Belfast or Bosnia or Beirut or Baghdad."

Hell, no

What Putnam and Campbell set out to establish with American Grace is that people in this country, at least in personal, private terms, are unusually tolerant. Then they tried to figure out why, via what Putnam calls "the most comprehensive and in-depth survey of the role that religion has played in American life in the last half-century."

As described on the book's website, the researchers conducted two phone surveys a year apart. The first, in 2006, asked 3,100 people questions about "their religion (beliefs, belonging and behavior) and their social and political engagement." The next year, the researchers followed up with as many of their original interview subjects as they could find, with a similar list of questions. (Copies of the surveys are available at americangrace.org.)

With the data collected, it turned out the key behind this very unusual combination of being devout and diverse and still tolerant appears to be ... Aunt Susan.

In the metaphor, Aunt Susan doesn't belong to your religion. "And you know that your religion says that poor Aunt Susan is not going to make it to heaven," says Putnam.

"But on the other hand, you know Aunt Susan, and for sure Aunt Susan is going to heaven. Heaven was made for Aunt Susan. There is a contest between what our religion formally says, and Aunt Susan. And Aunt Susan almost always wins."

Simply put, it's hard to believe that your Aunt Susan is going to burn in hell. And over the past 50 years, the number of Aunt Susans in American families has increased dramatically.

According to research by Campbell and Putnam, roughly one-third of Americans have switched religions in their lifetimes, and between one-third and one-half of all American marriages are interfaith.

Here's a catch, though: "Very few Americans today have a Muslim Aunt Susan," Putnam says. Which helps to explain the call by Southern pastor Terry Jones to burn the Quran, and the pre-election hysteria surrounding the construction of an Islamic community center in lower Manhattan.

"The popular commentary on the mosque issues sort of assumes that the reason that we don't like Muslims or are intolerant of Muslims is because of terrorism and all that," Putnam says. "And I don't deny that that's a little bit of the story. But if you look carefully, Americans are almost as negative toward Buddhists as we are toward Muslims. And as far as I know, there are no Buddhist suicide bombers."

Poles and polls

In America, with religious familiarity comes tolerance, he says. It's the fundamental message of the book.

"But the larger picture that we are trying to convey: We think that the public discussion of the cultural wars has vastly exaggerated how hostile the two poles of that debate are toward one another."

The evangelical right and the secular left, Putnam says, actually fear that the other is more hostile than it actually is. Most Americans are "a little cooler, on average" toward non-religious people and evangelicals than they are toward other groups, "because they are the two poles in this public debate."

"In the public arena, we have become more and more polarized. More and more of us seem to be either extreme right-wing evangelicals, or extreme secular progressives. And optically, those two poles have come to define for most people what it means to be either evangelical or secular."

Yet, surprisingly, when they interviewed the people in those opposing poles, they viewed themselves as tolerant and in the middle.

"What that means is, the politics are driving the polarization and not so much the religion," Putnam says. Of course, it is not 100 percent, "but the deeper and much more troubling polarization is the political one. And, gosh, religion was not a large part of the last political campaign. It was mostly about economics and not about gay marriage and abortion, but nevertheless, it was an exceptionally hostile and mean-spirited debate.

"And that says to me that nowadays, we are able to have a really hostile discussion even when religion isn't a part of it."